What is Kelly Criterion?

How the Kelly Criterion works

The formula is: stake percentage = (bp – q) ÷ b, where b is the decimal odds minus 1 (i.e. the net return per unit staked), p is your estimated probability of winning, and q is your estimated probability of losing (1 – p). The output is the fraction of your current bankroll you should wager. If the result is zero or negative, the formula says do not bet at all, because you have no edge.

Full Kelly can produce large swings in bankroll even when you have a genuine edge, because probability estimates are rarely exact. For this reason many bettors use a fraction of the Kelly recommendation — commonly half Kelly or quarter Kelly — to smooth out variance. Fractional Kelly sacrifices some theoretical growth rate in exchange for a more manageable drawdown. The formula does not account for bookmaker account restrictions, withdrawal delays, or minimum and maximum stake rules, so treat it as a guide rather than a precise instruction.

A worked example

Suppose a tipster advises a football selection at decimal odds of 3.00 (b = 2.00) and their verified strike rate suggests the true win probability is 40% (p = 0.40, q = 0.60). Plugging those numbers in: (2.00 × 0.40 – 0.60) ÷ 2.00 = (0.80 – 0.60) ÷ 2.00 = 0.20 ÷ 2.00 = 0.10, or 10% of bankroll. On a £1,000 bankroll that would mean a £100 stake. Half Kelly would reduce that to £50, quarter Kelly to £25.

Notice how sensitive the formula is to your probability estimate. If you revise the true win chance down to 35%, the calculation becomes (2.00 × 0.35 – 0.65) ÷ 2.00 = 0.05 ÷ 2.00 = 2.5% of bankroll — a dramatically smaller stake. Overestimating your edge is the most common error; it leads to overbetting and accelerated losses, which is why honest, long-run proofed records matter before applying Kelly to any tipster's selections.

Why it matters when judging or following a tipster

When reviewing a tipster, the Kelly Criterion is a useful lens for assessing whether their recommended stakes are proportionate to their demonstrated edge. A tipster advising flat 5% bankroll stakes on a service with a thin or unverified yield is implicitly asking you to overbet. Checking their historical yield and strike rate against a Kelly calculation tells you whether their stake guidance is mathematically defensible or simply aggressive. Sustainable tipsters generally recommend level stakes or conservative fractional Kelly precisely because it protects subscribers during inevitable losing runs.

From a follower's perspective, Kelly also highlights the importance of sample size and drawdown data. A short proofed record means your probability estimates for that tipster carry high uncertainty, which in Kelly terms argues for staking at a smaller fraction still — perhaps one eighth or one tenth of Kelly — until the record is statistically meaningful. Blindly applying full Kelly to a tipster with 50 proofed bets is high-risk regardless of their current profit figures. Use Kelly as a ceiling, not a default.

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